The present invention relates generally to the art of plastic pipe fittings for connecting tubing, pipe and the like. More specifically, the present invention relates to the construction of a “ribbed” pipe fitting for connecting large-diameter corrugated plastic pipe used in, and without limitation to, buried gravity-flow drainage, water storage, and sewage applications, particularly dual wall pipe having a smooth interior wall for handling increased load, deep fill or increased fluid flow capacity, with increased internal and external hydrostatic pressures. Although the present invention has particular relevance to the above applications, it will be appreciated that the principles of the present invention may also find application in other types of pipe and tubing configurations where structural performance or watertight integrity are especially critical.
For purposes of the present invention, it is important to note the difference between pipe and pipe fittings having a “ribbed” wall structure profile and those having an outer “corrugated” wall structure profile. The primary difference is that a “ribbed” wall structure profile is solid and homogeneous throughout, whereas a “corrugated” wall structure profile is defined by a relatively thin outer wall structure that follows the contour and forms each corrugation. Thus, an open space is defined between opposing sidewalls of each corrugation, whereas each rib is solid throughout. Consequently, to achieve comparable structural performance, the formation of pipes and fittings having a ribbed wall construction will typically require a greater volume of material than pipes and fittings having corrugations of comparable size.
Historically, gravity-flow drainage pipe utilized for agricultural, residential, civil construction and recreational purposes was constructed of concrete, steel or clay. The use of plastic materials, however, such as high-density polyethylene (HDPE), has gained significant popularity over the years in the construction of in-ground corrugated tubing for use in gravity-flow water management and sewage applications. In high volume water management applications, such as storm sewers, highway drains and culverts, large diameter (12″-60″ dia.) “dual” wall corrugated plastic pipe is now typically utilized. Such dual wall pipe is characterized by having a smooth inner cylindrical wall to which an outer corrugated wall is attached. The corrugation profile of the outer wall typically has a relatively broad crown region and steep sidewalls, thereby defining a hollow interior between the corrugation and inner wall. This construction has been found to minimize material cost while at the same time maximize fluid flow and structural capability under buried load conditions. Examples of patents disclosing such dual wall corrugated pipe include Goddard, U.S. Pat. No. 6,644,357; Hegler, U.S. Pat. No. 5,071,173; Hegler, U.S. Pat. No. 4,779,651; and Bonnema et al., U.S. Pat. No. 4,913,473.
Most corrugated pipe in the HDPE pipe industry, including dual wall pipe, are now formed via a continuous in-line molding process, whereby each section of pipe is molded with an integrally-formed bell coupler that facilitates connection to an adjoining section of corrugated plastic pipe. For dual wall pipe, a two-stage process is used, whereby the outer corrugated wall is extruded first and the inner smooth wall follows, attaching itself to the outer corrugated wall.
Corrugated plastic pipe fittings, on the other hand, such as tees, wyes, elbows, bends, etc., are not typically molded in-line. In fact, for large-diameter corrugated pipe, hand fabricated (i.e., non-molded) fittings have become the standard for use due to the relatively low entrance costs and related manufacturing difficulties. Such fittings are typically fabricated by cutting a smaller section of corrugated pipe to mate with an adjoining section, and then hand or machine welding the two together at their seam to form the desired fitting. For instance, to form a Tee fitting, a circular aperture is cut into the side of the first section of corrugated pipe, and then a second section of pipe is cut and fitted to mate with the opening of the first pipe. Thereafter, the two sections of pipe are welded together at their seam. Separate coupler sections (typically bell couplers) are then welded to the ends of each of the pipe sections in the same manner Consequently, multiple welded seams are required to form such a fabricated pipe fitting.
While some attempts have previously been made to mold plastic corrugated pipe fittings at smaller diameters (typically 10″ or less), for a number of reasons, this has not been the case for large-diameter pipe fittings. First, smaller diameter pipe does not typically experience the significant loading, and resultant levels of material stress and strain experienced by large diameter pipe. Consequently, the design, service, and material requirements are less stringent, and small diameter molded fittings can be produced more economically.
Secondly, as pipe diameters increase, difficulties in the molding process also increase, particularly for pipe fittings having a smooth interior with ribbed or corrugated exterior configurations. For instance, with injection molding, the core of the mold must include a “draft angle” to permit proper separation of the mold from the molded part. At larger diameters, this draft angle has an exaggerated effect resulting in a constriction of the interior of the molded fitting, thus adversely affecting the hydraulic performance of the pipe. Such large-diameter molds are also extremely expensive to manufacture, as the molds must be formed of solid steel to withstand the high pressure during the injection process.
Blowmolding is also generally unsuitable for fittings having a smooth interior, as it uses internal pressure to essentially stretch an extruded sheet of plastic to the desired shape. Thus, the molded pipe fitting takes on the shape of the exterior surface of the mold (i.e., corrugated), and it becomes very difficult to form a smooth interior. Increasing wall thickness in attempt to fill corrugations and create a smooth interior “ribbed” wall structure can lead to bridging and other defects caused by attempting to bend the thicker material around tight corners and through narrow passageways.
The use of rotational molding (i.e., process of heating and rotating a hollow mold to melt and disperse imported plastic material against the inner surface of the mold for subsequent cooling and formation of a product) has also been discouraged in the formation of solid ribbed structures, due in large part to manufacturing difficulties, geometry constraints and costs involved. While generally considered unsuitable for forming smaller fittings due to cycle times, geometry requirements, and material costs, rotational molding does provide a possible viable option for manufacturing large-diameter pipe fittings, where increased profile geometries tend to be more desirable for use in buried applications where watertight and/or increased structural integrity is necessary. If utilized at all, however, rotational molding has in past history been typically limited in use to the formation of relatively thin single-walled structures having hollow corrugations or solid ribs with shallower, broader profiles, which are not suitable for use in deep fill and heavy load conditions.
Consequently, it has long since been generally well accepted in the HDPE corrugated pipe industry that the mold and material costs, and design challenges associated with implementing rotational molding of large-diameter structural pipe fittings with smooth interiors was simply too great. Since it is not presently practical to mold dual wall fittings individually with a smooth interior and outer “corrugated” wall structure, any such molded fitting must be constructed with a solid ribbed profile. Consequently, the belief in the industry has been that the challenges associated with the design and material cost of the fitting would be prohibitive in comparison to fabricating such fittings by hand from leftover sections of corrugated tubing. For this reason, the industry has for many years continued to rely upon the use of hand fabricated fittings for large diameter corrugated tubing.
Although functional, these hand fabricated fittings do come with a number of limitations and drawbacks. Since fabricated fittings are manufactured separately by hand, there is no modularity in design, and their consistency can vary, sometimes significantly, from a number of variables including personnel, equipment settings, method of fabrication, etc. This is readily seen since fabricated fittings are made from flat stock which requires molding pipe from resin, shipping the pipe sections to and from a fabrication facility, with additional cutting and preparation of components for welding thereafter.
Furthermore, the welding process itself is complicated and can lead to inconsistencies and performance deficiencies. The failure modes for most fabricated fittings occur mainly at the welded joint either as a leakage or structural failure. These inherent inconsistencies, deficiencies, and multiple steps of fabrication lead to performance variations/limitations, especially as it relates to the watertight and structural performance of the fabricated fittings. Burial depth and structural performance of fabricated fittings are generally much less than that of standard corrugated pipe. For example, maximum cover for a fabricated fitting is typically less than ten feet 10′ as compared to thirty-five feet plus (35′+) for the same diameter HDPE corrugated pipe under similar circumstances. Also, due to the welding process, watertight integrity is always suspect, and one fitting can easily cause enough leakage in a tested watertight system to result in significant liability and cost.
It is therefore apparent that there is a significant need in the industry, particularly as concerns large-diameter corrugated HDPE pipe, for a cost efficient moldable large-diameter pipe fitting which is modular in design and has a seamless (i.e., not hand-fabricated) rigid body that has the structural stability capable of withstanding similar loading conditions as large diameter corrugated HDPE pipe, and which is so constructed as to provide increased watertight integrity and structural performance.